In recent years, the idea that acid rain levels help control those of dissolved organic matter (DOM) has gained traction because organic matter dissolves more easily as rainwater’s acidity decreases. But researchers have found other confounding correlations with DOM levels. For example, rising carbon dioxide can lead to additional plant growth; the extra plant matter in turn could break down to produce DOM that enters lakes (Environ. Sci. Pollut. Res., DOI: 10.1007/s11356-009-0176-7).

SanClements, Gretchen Oelsner of the Environmental Protection Agency, and their colleagues realized that historical water samples collected by the EPA might offer some answers. They wanted to compare the relative abundances of two types of DOM over time: terrestrial organic matter, which comes from land plants and soils, and lake organic matter, which comes from aquatic microbes and algae. If changes in rain acidity caused the increase in overall DOM levels, then the researchers expected to find a shift toward the terrestrial type because rainwater would carry more land-based organic matter into lakes.

To test the hypothesis, the team used fluorescence spectroscopy to analyze water samples from nine Maine lakes dating from 1993 to 2009. Since each DOM type has unique fluorescence signals, the researchers could determine the types’ relative abundance. They found that as total organic matter levels increased over time, the terrestrial signal grew stronger, supporting the idea that less acid rain led to more DOM.